1. Field of the Invention
The present invention relates to a fixing device for fixing an image on a recording medium, and an image forming device including the fixing device.
2. Description of the Related Art
In an image forming device such as a copier, a printer, a fax machine, or an MFP having a function of these devices, and the like, a copied matter and a recorded matter may be obtained by heating and fixing an unfixed image that is transferred and is being carried on a recording medium such as a sheet of paper.
In the fixing process, a developer, in particular, toner included in an unfixed image is melted and softened and penetrated into a recording medium by heating the unfixed image carried by the recording medium while sandwiching and conveying the recording medium by a fixing member and a pressing member, thereby the toner is fixed onto the recording medium.
As a fixing heater control method, a soft start/stop control is conventionally known, and is described in Japanese Patent Application Laid-open No. 2002-182521. In a soft start control, in order to limit the inrush current at the initial time period after turning on the heater, the turn-on period of heater is gradually increased, and once the filament of the heater is heated up to a certain degree, the heater starts to be controlled such that the turn-on period of heater is maintained constant. On the other hand, in a soft stop control, the turn-off period of heater is gradually increased before finally turning off the heater. The soft start and the soft stop are together called the soft start/stop control. The soft start/stop control also includes, instead of the increasing of the turn-on/turn-off period of heater, gradual narrowing of interval between the turn-on period and the turn-off period with both periods held constant.
Now, at the time of heating a fixing member up to a predetermined temperature by a heat source, if the heating time up to the predetermined temperature is sufficiently short, omitting a preheating step in a standby state can significantly reduce energy consumption without greatly affecting the usability of a user. To achieve this point, in recent years, a low heat capacity member, such as a thin roller, a thin belt formed from a metal base and an elastic rubber layer, or the like is increasingly being used as the fixing member. Also, with respect to a heat source, rapid heating is realized by using, besides a halogen heater that heats a fixing member by radiation heat, a ceramic heater, an IH method with high heating efficiency or the like. Fixing devices having such structures are disclosed in Japanese Patent Application Laid-open No. 2007-79040, Japanese Patent Application Laid-open No. 2010-32625, Japanese Patent Application Laid-open No. 2007-334205 and Japanese Patent Application Laid-open No. 2008-129517, for example.
With a fixing device that uses a fixing member of a low heat capacity as described above, the amount of heat stored in the fixing member is small, and thus, the temperature of the surface of the fixing member is easily changed at the time of fixing the toner. That is, the heater turn-on state at the instant of a sheet of paper passing through is likely to reflect the temperature state of the fixing member. Accordingly, the difference between a required duty of a heater according to a soft start/stop and an actual turn-on duty which is allowed in a conventional fixing device may not be allowed (in a fixing device which uses the fixing member of a low heat capacity), and there is a problem of poor fixation such as a cold offset, a hot offset or the like. Meanwhile, in the present specification, “duty ratio” will be referred to as “duty”.
Furthermore, there is also a problem that, because the required duty, which is obtained by means of duty calculation based on the temperature information of a fixing member detected by a temperature detection unit, and the actual turn-on duty are different, if an amount of heat generation in the actual turning-on is too much, the temperature may rise until a safety device activates. Once the safety device activates, it cannot be deactivated by a user, and a service man has to be called, which results in downtime and lowered productivity.
Therefore, there is a need for a fixing device and an image forming device including the fixing device that are capable of maintaining an appropriate state where poor fixation does not occur even when a soft start/stop control is performed and where temperature can be prevented from getting out of control.